Embodiments of the present invention related generally to the field of medical devices and methods, and in particular to therapeutic modalities involving tissue ablation or lesion formation.
Atrial fibrillation (AF) can refer to a heart beat rhythm disorder (or “cardiac arrhythmia”) in which the upper chambers of the heart known as the atria quiver rapidly instead of beating in a steady rhythm. This rapid quivering reduces the heart's ability to properly function as a pump. AF is a common clinical condition, and presents a substantial medical issue to aging populations. AF is costly to health systems, and can cause complications such as thrombo-embolism, heart failure, electrical and structural remodeling of the heart, and even death. Relatedly, AF typically increases the risk of acquiring a number of potentially deadly complications, including thrombo-embolic stroke, dilated cardiomyopathy, and congestive heart failure. Quality of life is also impaired by common AF symptoms such as palpitations, chest pain, dyspnea, fatigue and dizziness. People with AF have, on average, a five-fold increase in morbidity and a two-fold increase in mortality compared to people with normal sinus rhythm. One of every six strokes in the U.S. (some 120,000 per year) occurs in patients with AF, and the condition is responsible for one-third of all hospitalizations related to cardiac rhythm disturbances (over 360,000 per year), resulting in billions of dollars in annual healthcare expenditures. The likelihood of developing AF increases dramatically as people age; the disorder is found in about 1% of the adult population as a whole, and in about 6% of those over age 60. By age 80, about 9% of people (one in 11) will have AF. According to a recent statistical analysis, the prevalence of AF in the U.S. will more than double by the year 2050, as the proportion of elderly increases. A recent study called The Anticoagulation and Risk Factors in Atrial Fibrillation (ATRIA) study, published in the Spring of 2001 in the Journal of the American Medical Association (JAMA), found that 2.3 million U.S. adults currently have AF and this number is likely to increase over the next 50 years to more than 5.6 million, more than half of whom will be age 80 or over.
As the prevalence of AF increases, so will the number of people who develop debilitating or life-threatening complications, such as stroke. According to Framingham Heart Study data, the stroke rate in AF patients increases from about 3%/year of those aged 50-59 to more than 7%/year of those aged 80 and over. AF is responsible for up to 35% of the strokes that occur in people older than age 85. Efforts to prevent stroke in AF patients have so far focused primarily on the use of anticoagulant and antiplatelet drugs, such as warfarin and aspirin. Long-term warfarin therapy is recommended for all AF patients with one or more stroke risk factors, including all patients over age 75. Studies have shown, however, that warfarin tends to be under-prescribed for AF. Despite the fact that warfarin reduces stroke risk by 60% or more, only 40% of patients age 65-74 and 20% of patients over age 80 take the medication, and probably fewer than half are on the correct dosage. Patient compliance with pharmacological intervention such as warfarin is problematic, and the drug requires vigilant blood monitoring to reduce the risk of bleeding complications.
More recently, the focus has shifted toward surgical or catheter ablation options to treat or effect a cure for AF. The ablation techniques for producing lines of electrical isolation are now replacing the so-called Maze procedure. The Maze procedure uses a set of transmural surgical incisions on the atria to create fibrous scars in a prescribed pattern. This procedure was found to be highly efficacious but was associated with a high morbidly rate. The more recent approach of making lines of scar tissue with modern ablation technology has enabled the electrophysiologist or cardiac surgeon to create the lines of scar tissue more safely. Ideally, re-entrant circuits that perpetuate AF can be interrupted by the connected lines of scar tissue, and the goal of achieving normal sinus rhythm in the heart may be achieved.
Electrophysiologists often classify AF by the “three Ps”: paroxysmal, persistent, or permanent. Paroxysmal AF, typically characterized by sporadic, usually self-limiting episodes lasting less than 48 hours, is usually the most amenable to treatment, while persistent or permanent AF can be much more resistant to known therapies. Researchers now know that AF is a self-perpetuating disease and that abnormal atrial rhythms tend to initiate or trigger more abnormal rhythms. Thus, the more episodes a patient experiences and the longer the episodes last, the less chance of converting the heart to a persistent normal rhythm, regardless of the treatment method.
AF is often characterized by circular waves of electrical impulses that travel across the atria in a continuous cycle, causing the upper chambers of the heart to quiver rapidly. At least six different locations in the atria have been identified where these waves can circulate, a finding that paved the way for maze-type ablation therapies. More recently, researchers have identified the pulmonary veins as perhaps the most common area where AF-triggering foci reside. Triggers for intermittent AF and drivers for permanent AF can be located at various places on the heart, such as the atria. For example, where triggers or drivers are located near the pulmonary veins, it follows that treatment may involve electrical isolation of the pulmonary veins. Technologies designed to isolate the pulmonary veins or ablate specific pulmonary foci appear to be very promising and are the focus of much of the current research in catheter-based ablation techniques.
There are many instances where it is beneficial to perform a therapeutic intervention in a patient, using a system that is inserted within the patient's body. One exemplary therapeutic intervention involves the formation of therapeutic lesions in the patient's heart tissue to treat cardiac conditions such as atrial fibrillation, atrial flutter, and arrhythmia. Therapeutic lesions may also be used to treat conditions in other regions of the body including, but not limited to, the prostate, liver, brain, gall bladder, uterus, and other solid organs. Typically, the lesions are formed by ablating tissue with one or more electrodes. For example, certain cardiac surgical procedures involve administering ablative energy to the cardiac tissue in an attempt to create a transmural lesion on the tissue. Although cardiac ablation devices and methods are currently available and provide real benefits to patients in need thereof, many advances may still be made to provide improved devices and methods for creating lesions in cardiac tissue to treat AF and other arrhythmias. Embodiments of the present invention provide solutions to at least some of these outstanding needs.